Here metum is almost equivalent to “the terrors of the law,” and hence propter is used, though the ablative would be more natural.

c. The ablatives causā and grātiā (for the sake of) are used with a Genitive preceding, or with a pronoun in agreement.

eā causāon account of this

Quā grātiā? (Ter. Eun. 99)for what purpose?

meā causāfor my sake

meā grātiā (Plaut.)for my sake

ex meā et reī pūblicae causāfor my own sake and the republic's

praedictiōnis causā (N. D. 3.5)by way of prophecy

exemplī grātiā (verbī grātiā)for example

suī pūrgāndī grātiāfor the sake of clearing themselves

Note—But grātiā with possessives in this use is rare.

Footnotes

1. The cause, in the ablative, is originally source, as is shown by the use of ab, dē, ex; but when the accusative with ad, ob, is used, the idea of cause arises from nearness. Occasionally it is difficult to distinguish between cause and means (which is the old instrumental case) or circumstance(which is either the locative or the Instrumental).